Wells typically contain sub-surface safety valves (SSV), which are actuated from the surface through a control line, which runs down to the valve. These valves have a biased closure member, known as a flapper. The flapper is biased into contact with a mating seat for isolation of a zone in the well from the surface. The flapper is positioned perpendicularly to the longitudinal axis of the wellbore, when it is in the closed position. To open the valve, pressure through the control line causes a flow tube to shift against a bias force. The flow tube engages the flapper to rotate it 90 degrees. The flow tube continues to advance as the flapper is positioned behind it.
In certain wells, with the SSV closed and formation pressure acting on the flapper in the closed position, it is desirable to equalize the pressure on both sides of the flapper before attempting to rotate it with the flow tube. A pressure imbalance can occur because there is gas at low pressure above the flapper and high pressure from the formation below the flapper. One costly way to equalize the pressure is to add heavy fluid above the flapper. An easier way is to install and equalizing valve in the flapper so that when the flow tube starts moving down it strikes the plunger of the equalizing valve first. This causes the plunger to move to equalize the pressure across the flapper before the flapper is pushed away from its seat by the flow tube. A few examples of this design are U.S. Pat. Nos. 4,475,599 and 4,478,286.
The layout of the principal components of an SSV in the closed position is illustrated in FIG. 1. The SSV 10 has a body 12 and a flapper 14 pinned at pin 16 to body 12. The flapper 14 is biased to the closed position shown by a spring 18. Flapper 14 is in contact with a seat 20, in the closed position shown in FIG. 1. A flow tube 22 is driven by pressure in a control line (not shown) against the force of a spring 24. The equalizer valve 26 is disposed in the flapper 14 so that upon initial downward movement of the flow tube 22, the initial contact occurs between the equalizer valve 26 and the flow tube 22, which results in pressure equalization before the flow tube 22 pushes the flapper 14 off of seat 20. When the flow tube 22 moves down completely, as shown in FIG. 2, the flapper 14 is behind the flow tube 22. FIG. 2 also illustrates the initial position of equalizer valve 26 when the flapper 14 is in the closed position of FIG. 1. It can be seen that the equalizer valve is engaged off-center by the flow tube 22. One reason for this offset contact is the limited choice of placement of the equalizer valve 26. FIG. 3 shows a view of the underside of the flapper 14 showing the bore 28 located in the thick segment 30 of flapper 14. In order to get a sufficiently long bore, it was located in the remotest part of the thick segment 30. FIG. 6 illustrates the need for offset contact. The equalizer valve 26 comprises a plunger 32 and a bore 34 that extends from the upper end 36 to lateral bores 38. When depressed by the flow tube 22 the lateral bores 38 extend below the lower end 40 of the flapper 14 and equalizing flow is established. The offset contact is used in this design to avoid obstructing the bore 34 during initial movement of the plunger 32. FIG. 2 illustrates another aspect of the prior design. The plunger 32 had a chamfer 42 so as to avoid contact with the flow tube 22 when the SSV was in the open position. This need for clearance made the end of the plunger 32 asymmetrical, making the installed orientation critical to achieve the desired clearance with the flow tube 22 when the SSV was opened.
The offset contact between the flow tube 22 and the plunger 32 tended to put a counterclockwise moment on the plunger 32 and resulted in abnormal wear on portion 44, closest to the point of offset contact. To combat this problem of wear, the plunger 32 was first produced and measured. Thereafter, bore 28 was machined to about 0.001 inch over the diameter of the plunger 32 and both surfaces were polished to 8 RMS. The problem was that each plunger 32 was custom fit to each bore 28 so that it was not possible to maintain a store of spare parts that could be counted on to provide adequate service. Even with expensive machining, the problem of premature wear due to offset contact, created a reliability and maintenance concern. Accordingly, the objective of the present invention is to provide design features to minimize or otherwise cope with the wear issue from offset contact. Another feature of the invention is to work around offset contact that caused the wear and still allow the equalizer valve 26 to effectively function. Those skilled in the art will appreciate how the invention addresses these objectives from a review of the detailed description of the preferred embodiment, which appears below.